Arc-over protective system for high-frequency heating systems



April 0, 1951 J E WALSTROM 2,548,246

ARC-OVEE PROTECTIVE SYSTEM FOR HIGH-FREQUENCY HEATING SYSTEMS Filed Dec. 11, 1946 2 Sheets-Sheet l u E 5 q' & 7 Q g? w w LE L ..E u

I\ @l E INVENTO/P J0///Y 5. 14/545720? v BY Wa /w,

A TTORNEYS April 10, 1951 J. E. WALSTROM 2,543,246

. ARC-OVER PROTECTIVE SYSTEM FOR HIGHJREQUENCY HEATING SYSTEMS ATTORNEYS Patented Apr. 10, 1951 ARC-OVER PROTECTIVE SYSTEM FOR HIGH-FREQUENCY HEATING SYSTEMS,

John E. Walstrom, Middlesex County, Mass., as-

signor to The Girdler Corporation, Louisville, Ky., a corporation of Delaware ApplicationDecember 11, 1946, Serial No..715,5.22

' Claims.

This invention is; concerned, with the protection of dielectric loads from damage due to formation of an arc between the heating electrode and with the protection of the high-frequency power tubes used for generating the high-frequency dielectric-heating power from damage due to insufiicienoy of their high-frequency excitation.

In, high-frequency dielectric heating systems, the dielectric to be heated is disposed between or adjacent metal plates forming part or all of the capacity of the tank circuit of a high-frequency power oscillator. In operation of such systems, an are sometimes may form between the load capacitor plates, with the result that the work is burned or damaged to serious degree. Also, a usual result of such arc-over across the load is that the RF plate voltage decreases, with resultant decrease in grid excitation due to re duced feed-back. Such decreased grid excitation may result in such inefficient operation of the tube as to cause damage to the tube due to excessive plate dissipation. Such undesirable efiects may occur even though the anode current, because of continued feeble oscillations, may not increase sufficiently to cause operation of the anode current overload relay usually provided.

In accordance with the present invention, any are which forms between the electrodes is promptly extinguished by discontinuance of the supply of anode current to the oscillator in response to decreasebelow a predetermined mag nitude' of its high-frequency grid excitation. In addition to protecting the work, the interruption of supply of, anode current protects the tube from damage due to excessive anode dissipation arising from thearcing between the load plates or from other conditions causing insufiicient grid excitation. More specifically, there is provided a controlv tube, which need be only a small tube of the radio-receiver type, to whose control grid is. applied a biasing vcltagederived from the rectified high-frequency excitation of the main oscillator tube. Upon abnormal decrease of the grid excitation of the oscillator tube, the resulting change in the bias of the. control tube causes actuation of a, normally closedrelay in the anode circuit of thecontrol tubeto deenergize the anode power supply of the high power oscillator tube,

Preferably, the biasing voltage for the control tubeis deriveddirectly irornthe gridjbias resistor of the, oscillator tube so that under-excitation protection may be added without modification of the, oscillator circuit.

Further in accordance, with, the invention, to

permit initiation of a heating cycle without need for use of manual hold-in switches; or timing relays to disable the under-excitation protective system until after the oscillator tube has had opportunity to start functioning, the control tube itself, or the constants of its associated power supply, provide sufficient time delay to allow the oscillator to start and build up the control tube bias in readiness for any failure of the highfrequency excitation of the oscillator.

The invention further resides in systems having the, features of combination and arrangement hereinafter described and claimed.

For more detailed understanding of the invention, reference is made to the accompanying drawings in which:

Fig. 1 schematically illustrates one form of the protective system as incorporated in a dielectric heating system; and

Fig. 2 illustrates a modification of the protective circuit of Fig. 1.

Referring to Fig. l, the tube Ill is a high power tube suited to supply high-frequency energy to a tank circuit comprising the inductance H and. the capacitor l2 formed by spaced electrodes l3 and I4 between or adjacent, which is, disposed the dielectric material or loadto be heated. More specifically, the electrodes of the load capacitor [2 comprises an upper metal plate, I3 and a lower plate or tray [4 slidable into and out of a housing or cabinet (not shown) for the circuit components later described. In Fig. 1, the tray I4 is in the loading position in readiness for placement thereon of dielectric preforms or otherobjects or material to be heated. After loading, thetray is moved to the left to initiate a heating cycle, as later described.

In the particular oscillator circuit shown, the anode A of, tube It! is connected to the positive terminal of a high-voltage direct-current power supply [5 through" a radio frequency choke I6 between whose cold end and the chassis or ground of the unit is connected a bypass condenser ll of low impedance to high-frequency currents. The blocking condenser it connected between the anode A. of tube H3 and the tankcircuit inductance H effectively connects them so far as high, frequencies are concerned but prevents passage of the high voltage direct-current from the source IE to the tank circuit components.

The tube 10, as shown, rnay be of the directlyheated cathode type whose, cathode or heater K may be supplied with current by a step-down transformer 19 whose primary is connected to 3 a suitable available source of alternating current. The secondary circuit of transformer 19 is connected to the negative terminal of the highvoltage supply 15 through a filter choke 22, an ammeter 2| and the coil 22 of an overload relay 23.

The radio frequency portion of the grid circuit of the tube It] 'comprises an inductance 24, one terminal of which is connected to the grid G of tube In and whose other terminal is effectively at ground or cathode potential so far as radio frequency currents are concerned by virtue of the bypass condenser 25. In the particular oscillator shown, the grid circuit is of the so-called untuned type, though the inductance 24 is selected or adjusted so that in association with distributed capacities of the circuit it is resonant to a somewhat higher frequency than the frequencies to be generated by tube Ill. The coupling between the grid and plate circuits ofthe tube, required for generation of high-frequency oscillations, in the illustrated exemplary embodiment is provided by the inherent capacity between the grid and anode elements of the tube itself, although in some other types of oscillators with which the present invention may be employed, magnetic coupling of course may be utilized.

In shunt to the bypass condenser 25, there is the usual grid resistance comprising, in the circuit shown, the resistors 25, '21 and 28, the milliammeter 29, and the coil 30 of a relay 3| provided to protect the tube from excess grid current. The variable resistor 32, in shunt to the relay coil 30, is provided for adjustment of the tripping point of the relay.

When the tube In is oscillating, there is gridcircuit rectification of the high-frequency current supplied thereto from the anode circuit, and in consequence direct-current flows in the grid circuit through the resistors 262'B, ammeter 29 and relay coil 30. The flow of current is in such direction that it biases the grid G of tube ii! negatively with respect to the cathode K. The grid bias voltage so derived varies with different operating conditions of the tube: when the anode A is at high radio-frequency potential, the rectified grid current is correspondingly large and a substantial negative bias is supplied to the grid of the tube, whereas when the tube is not oscillating there is no grid current and the biasing potential is zero. When the high-frequency potential of the anode is excessively high, the rectified grid current flowing through overload coil 33 is abnormally large and the relay 3! is efiective to interrupt supply of power to the anode circuit of the tube, as later described. Conversely, when the tube is not oscillating, the anode current flowing through coil 22 rises to a high value, due to loss of grid bias, and the relay 23 is effective to interrupt further supply of anode current to the tube l0. However, these protective features have failed to meet contingencies encountered under actual condition of operation of dielectric heating systems. For example, and as above briefly referred to, in event of arcing between the plates l3 and M of the load capacitor I2, the tube may continue to oscillate, though feebly, so that the mean value of the anode current may not rise suificiently high to cause actuation of the relay 23, although, in fact, because of the reduced anode-circuit eificiency the power dissipated by the anode A of the tube may be greatly in excess of its safe or rated value. It is one of the principal purposes of the present invention to protect the tube in event of such con- 4 tingencies, as well as of others which result in under-excitation of the grid circuit of the tube 10.

To afford under-excitation protection, there is provided the control tube 33 which may be a small, receiver-type tube, such as a 6J5, having an anode a and whose grid 9 and cathode is are connected across a portion of the grid bias resistance of tube it; specifically, the cathode is of tube 33 may be connected to the chassis or to other point of the same potential as the cathode of the oscillator tube I0, and the grid 9 of the control tube 33 may be connected to the grid end of the resistor 28 which forms part of the grid bias resistance of the oscillator. In the anode circuit of the control tube 33 is included the coil 34 of relay 35 in shunt to which there is connected a capacitor 36 of substantial value for reasons later discussed. The anode current of tube 33 is supplied by a transformer 31 whose primary is connected to a suitable available source of alternating current. The control tube 33 also serves as a rectifier, avoiding need between the transformer 31 and the control tube of any additional rectifier tube and filter network.

The circuit parameters, specifically the eilective values of grid resistor 28 and the voltage applied between the anode and cathode of tube 33, are so chosen or adjusted that so long as the grid-excitation of tube HI is above a safe minimum, the negative potential applied to the grid 9 of the control tube 33, as derived from the rectified grid current of the oscillator tube II], is sufficiently large to hold the plate current of the control tube below a magnitude sufficient to effect actuation of relay 35, whereas a drop in negative potential applied to the grid of the control tube, such as would result from a drop in rectified grid current of the oscillator tube below a safe minimum value, will cause rapid increase in current in the anode circuit of the control tube. It is thus insured that when the highfrequency excitation of the oscillator tube falls below a safe minimum, there is extremely rapid operation of relay 35. This action of the protective system of the present invention is not attainable by energization of relay 35 directly from the grid current of the power tube H1, since in such latter case the grid current would be slowly decreasing and a slowly decreasing relay current will make uncertain the exact drop-out value thereof. Even more important, however, is the fact that in the protective system of the present invention small change in excitation of tube I0 is greatly amplified by the control tube 33, resulting in a large change in current flow in the circuit of the coil 34 of relay 35. This assures immediate and positive action of the relay when the grid excitation of tube I ll drops to and just below the minimum safe value. A further advantage of the disclosed protective arrangement is that the tube 33 and its circuit serves somewhat in the manner of an impedance matching arrangement so as to permit employment of an extremely sensitive relay, whereas if a relay were employed directly in the grid circuit of tube Ill, it would either have to be of a relatively heavy duty type, which would be inherently sluggish and insensitive, or a sensitive relay heavily shunted.

Should an arc form between the load capacitor plates l3 and H3 and the tube continue to oscillate feebly, the relay 23 may not operate, but the under-excitation of the grid circuit of tube I!) will cause substantial decrease of the rectified grid current traversing resistor 28. In conseesteem quence; the'grid'g oftube 33. assumes: atless: nega= ti-ve'value and the anode current ofitube'33 flowing'through the relay coil 34-becomes sufiicient' to cause the relay contact 38 to open, and as later-specifically described, thus causes an interruption of supply of anode current to the power tube I0.

For purposes of further explanation, it is assumed that the heater or filamentcircuits of all tubes of the system, including the oscillator tube II), the control tube 33 and rectifier tubes (not shown) included in the supply source l5, have previously been energized, and the system is in readiness for imitation of a heating cycle. Since these heater circuits are not involved. in the circuits to be.described, they therefore: areomitted from the drawings. At this time; the contacts 38, 39 and'49 of relays 35; 31' and 23, respectively; are closed. Thesafety orinterlock switch 41 and the contacts 42 and 4'! of relay 43 are also closed; There is thus comp eted a: circuit for energizing the coil 44 of relay 43 from the supply conductors 4546. The contact 41 of relay 44 is in circuit with winding 48 of the power relay 49, but this control circuit through coil 48 is not completed i.

until the tray electrode 14 is pushed to the left to rock the contact 56 out of engagement with fixed contact 5| and into engagement with fixed contact 52. Thus, upon movement of the load tray to heatingpositiomthe relay 49 is energized, :2:

whereupon its contact 53 moves to complete connectionof the high voltagesupply l5 forthe oscillator tube to a suitable source of alternating current. As will be understood; thecircuitcompl'eted by contact53 is that'of the primary of a step-up transformer in whose secondary circuit is included a suitable rectifier. Concurrently with the resulting application of high direct-current voltage to the anode A of'the' oscillator tube [0, the contact 54 of relay 49 completes the anode-circuit of the control tube; 33.

At the instant of application of' high voltage to the anode-circuit of tube 10, it is not in oscillating condition, and. should the control tube 33 at that time be in the same state as it must subsequently be. for under=excitation prevention, the oscillator would never have opportunity to start. However, the control circuit has a sufficientlv large time constant, as provided in Fig. l by the large condenser 36 in association with resistance 55, representative of the total anodecircuit resistance and including that of tube 33, to allow'the oscillator l9 opportunity to start oscil ating before the control tube assumes its normal operating condition. The time constant, through greater than that of the filter network associated with. the supply'source l5 and" includ ing the choke inductance and the inherent time delay in starting of'oscillations' by the tube I0, need'not be great and maybe only of the order of a small fraction of a second. In any event, the necessary time delay is readily empirically determined for any particular oscillator and power supply.

In event of arcing between the load capacitor plates or of other cause of under-excitation of the grid circuit of tube l0, the control tube 33 is at once efiective through the relay; and its contacts 38 to open the control circuit between conductors and 45 including, the coil 44 of relay 43. Upon deenergization of relay 43, its contact 4! opens to deenergize the coil 48 of'relay 49, whereupon contacts 53 and 54 of relay 49 interrupt the anode supplies of the oscillator tube and the control tube. Damage to the-work.

such. as: plastic: preforms, or: other: material. on:

trayrl4, is:minimizedfand damage .to tubel0 and associated. equipment, such. as. would otherwise result; by continued supply of. anodercurrent; to the: tube; while. operating with low anode-circuit eificiency, is: prevented;

After. thecontrol circuit, through the contacts 38:, 3.9, 48. 4!..42 and 41, has once been broken. by actuation of any of the protective relays, including relay 35, it cannot be re-establisheduntil the tray I4 has: been retracted to allow contact 59 of: the: tray switch to lie-engage its contact 5,! and seeifect re-energization of the coil 44 of .relay t3: After-relay coil. has been energized, its lockx-in contact 42'. closes, so that the relay43remains energizedwhen the.v loaded tray is again pushed in even thoughth-ere is separationof. 0.011? tacts 50 and 5| of the tray switch.

The-modification shownin Fig. 2 is in.all.respects'similar' to that of Fig; lexcept the time lag of the under-excitation. system, incident to .initiationof a heating cycle, is provided for or en. hanced by taking advantage of the greater time required to bring the cathode of a tube of the indirectly heated cathode type, such as a 6J5, up to emission temperature. More specfically, the heater h of the control tube 33 is supplied with current from the secondary Winding 56 of trans! former 3 1 in a circuit including the additional movable contact 51 of the power relay 49. Thus, when the relay 491s energized, as above described at the beginning of a heating cycle, although its contacts 53' and 54 are concurrently closed to apply anode voltage to the oscillator and control tubes, respectively, the control tube is not in conditon to function because its cathode is then cold and does not attain emission temperature until an appreciable time after closure of contact 5? in the heater circuit of the tube. It is thus assured the oscillator ID has opportunity to begin to function. However, the time delay so afiorded is not so great as to interfere with proper operation of the control circuit, including the control tube 33 and relay 3-5, in the event of unsafe drop in grid excitation of tube 19 shortly after commencement of oscillation of the latter.

In Fig. 2, the contact 54 of relay 49 may be omitted-as the anode circuit of the control tube is effectively open until the cathode is sufficiently heated subsequent to closure of contact 51 of'relay 4-9 which as above stated is in the heater circuit ofthe control tube.

Further description of Fig. 2 appears unnecessary, as the previous description of Fig. I is otherwise directly applicable thereto.

In both modifications or" the invention 'it will be observed the relay '35 is normally closed. Consequently, the failure of tube 33 or of any of the other components of the under-excitation protective system, or'removal of tube 33 from the system, will not disable the oscillator, which may continue to function under the control of overload relays 3i and 23. This feature is in contrast with normally open minimum-current or under-voltage relays which must be held closed until the current rises above the desired value and which disable the entire system if a coil thereof fails or if the circuit thereto should remain open.

It shall be understood my invention. is-not limited to the particular embodiments disclosed, but that changes and modifications may be made within the scope of the appendedclaims.

What is claimed is:

1. A system for protecting the. work load and the high-frequency power tube of a dielectric heating system from effects of arcing between the load-heating plates, comprising a control tube, a relay in the anode circuit thereof for controlling the anode-current supply of the power tube, and means for deriving from the high-frequency excitation of the power tube a direct current gridbiasing voltage for said control tube which except under sub-normal high-frequency excitation of said power tube is of magnitude precluding operation of said relay.

2. A system for protecting the work load and the high-frequency power triode tube of a dielectric heating system from eiTects of arcing between the load-heating plates, comprising a control tube, a negative grid-biasing circuit therefor including resistance traversed by the direct-current component of the grid current of said power tube, and a relay in the anode circuit of said control tube having contact structure controlling supply of anode current to said power tube and actuated upon reduction of the negative gridbias of said control tube incident to arcing between said load-heating plates.

3. A system for protecting the work load and the high-frequency power triode tube of a dielectric heating system from efiects of arcing between the load-heating plates, comprising a control tube, a negative grid-biasing circuit therefor including resistance traversed by the direct-current component of the grid current of said power tube, and a relay in the anode circuit of said control tube having normally closed contact structure actuated upon reduction of the negative gridbias of said control tube to efiect discontinuance of supply of anode current to said power tube upon occurrence of sub-normal grid excitation thereof.

4. A system for protecting the work load and the high-frequency power triode tube of a dielectric heating system from effects of arcing between the load-heating plates comprising a control tube, a relay in the anode circuit of said control tube having contact structure normally closed for supply of anode current to said power tube, and means including resistance traversed by the direct-current component of the grid current of said power tube for biasing said control tube to cut-off so long as the high-frequency excitation of said power tube is at or above a safe minimum and to cause rapid rise of the relay current to substantial magnitude insuring actuation of said contact structure to open position upon slight decrease of said excitation below its said safe minimum.

5. A system for protecting the work load and the high-frequency power tube of a dielectric heating system from effects of arcing between the load-heating plates comprising a control tube, a relay energizable concurrently to initiate independent functioning of the control tube and the power tube, a negative grid-biasing circuit for said control tube including resistance traversed by the direct-current component of the grid current of said power tube derived from its highfrequency excitation, and a relay in the anode circuit of said control tube having normally closed contact structure actuated upon reduction of the negative grid-bias of said control tube to deenergize the first-named relay.

6. A system for protecting the work load and the high-frequency power tube of a dielectric heating system from effects of arcing between the load-heating plates comprising a control tube, means for deriving from the high-frequency excitationof the power tube, a direct-current negative grid-biasing voltage for said control tube, ,a relay in the anode circuit of said control tube actuated upon reduction of said grid-biasing voltage to interrupt supply of anode current to said power tube, and means operable concurrently to initiate functioning of the power tube and the control tube, the time characteristic of the protective system itself aifording opportunity for the power tube to generate high-frequency oscillations before the protective system is effectively functioning.

7. A system for protecting the work load and the high-frequency power tube of a dielectric heating system from effects of arcing between the load-heating plates comprising a control tube, a relay energizable concurrently to apply anode voltage to the control tube and to the power tube, a negative grid-biasing circuit for said control tube including resistance traversed by the direct-current component of the grid current of the power tube derived from its high-frequency excitation, a relay in the anode circuit of said control tube having normally closed contact structure actuated upon reduction of the negative grid-bias of said control tube to deenergize said first-named relay, and a time-delay network in circuit with said control tube affording said power tube opportunity to generate high-frequency oscillatio-n after energization of said first-named relay and before said control tube is in readiness to function.

8. A system for protecting the work load and the high-frequency power tube of a dielectric heating system from efiects of arcing between the load-heating plates comprising a control tube of the indirectly heated cathode type, a relay energizable concurrently to apply anode voltage to the power tube and to energize the heater circuit of said control tube, the slow-heating of the cathode of said control tube permitting the power tube opportunity to generate oscillations before the control tube is in readiness to function, a negative grid-biasing circuit for said control tube including resistance traversed by the direct-current component of the grid current of the power tube derived from its high-frequency excitation, and a relay in the anode circuit of said control tube having normally closed contact structure actuated upon reduction of the negative grid-bias of said control tube to deenergize said first-named relay.

9. A system for protecting the work load and the high-frequency self-excited oscillator of a dielectric heating system comprising a control tube, a relay energized by the anode current of said control tube for controlling the anode-current supply of the oscillator tube, and means including res stance common to the direct-current paths from the grids to the cathodes of said tubes for app-lying to the grid of the control tube a direct-current bias of magnitude dependent'upon the high-freouency grid-excitation of the oscillator tube and effective except during sub-normal magnitudes of the high-frequency grid-excitation of the oscillator tube to preclude operation of said relay.

10. A system for protecting the work load and the tube of the high-frequency self-excited oscillator of a dielectric heating system comprising a control tube, resistance means common to the grid-cathode circuits of said tubes for applying to their grids direct-current biasing voltages derived from the high-frequency grid-excitation of the oscillator tube, and a relay energized by the anode current of saidcontrol tube and having 9 contact structure actuated upon reduction below normal of the grid-bias of said control tube to discontinue supply of anode-current to said oscillator tube upon occurrence of sub-normal grid excitation thereof.

11. A system for protecting the Work load and the tube of the high-frequency self-excited oscillator of a dielectric heating system comprising a control tube, resistance means common to the grid-cathode circuits of said tubes for applying to their grids direct-current biasing voltages derived from the high-frequency grid-excitation of the oscillator tube, a relay energized by the anode current of said control tube and having contact structure actuated in response to reduction below normal of the grid-bias of said control tube, and a second relay effective upon aforesaid actuation of the contact structure of the first-named relay to discontinue supply of anode-current to both of said tubes.

12. A system for protecting the work load and the tube of the high-frequency self-excited oscillator of a dielectric heating system comprising a control tube, a relay energizable concurrently to apply anode voltage to said tubes, a time-delay network in the anode circuit of said control tube to allow said oscillator tube to generate highirequency oscillations before said control tube is in readiness to function, resistance means common to the grid-cathode circuits of said tubes for applying to their grids direct-current biasing voltages derived from the high-frequency grid excitation of the oscillator tube, and a second relay energized by the anode current of said control tube and having contact structure movable in response to reduction below normal of the grid bias of said control tube to effect de-energization of the first-named relay for concurrent discontinuance of application of anode voltage to said tubes.

13. A system for protecting the work load and the tube of the high-frequency self-excited oscillator of a dielectric heating system comprising a control tube, a relay energizable concurrently to apply anode voltage to the oscillator tube and t the heater of the control tube, said oscillator tube thereby generating high-frequency oscillations before said control tube is in readiness to function, resistance means common to the gridcathode circuits of said tubes for applying to their grids direct-current biasing voltages derived from the high-frequency grid excitation of the oscillator tube, and a second relay energized by the anode current of said control tube and having contact structure movable in response to reduc- Numb er tion below normal of the grid-bias of said control tube to effect de-energization of the first-named relay.

14. A system for protecting the work load and the high-frequency oscillator tube of a dielectric heating system from effects of arcing between the load-heating plates comprising a relay having normally closed contacts separable to interrupt the anode circuit of said oscillator tube, means normally effective to maintain energization of said relay below contact-operating magnitude comprising a control tube and connections including said relay in the anode circuit of said control tube, and means effective upon arcing between said load-heating plates to produce rise of the relay ener ization above said contact-operating magnitude comprising resistance means in the grid-cathode circuit of said control tube and connections therefrom to an electrode circuit of said oscillator tube.

15. A system for protecting the work load and the high-frequency power tube of a dielectric heating system from effects of arcing between the load-heating electrodes comprising a relay having normally closed contacts separable to reduce supply of anode current to the power tube, a control tube and connection including said relay in its anode circuit, and biasing means for said control tube including resistance means in its grid-cathode circuit and connections therefrom to an electrode circuit of said power tube, the bias existent upon occurrence of said arcing permitting rise of the anode current of said control tube to magnitude effecting separation of said relay contacts.

JOHN E. WALSTROM.

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